The 'powered by thresholds' thing sounds more like a Feeding Restriction applied to some or all of their powers than a Catch, at least to me.
That's a fascinating way to do it. Mechanically, it fits far better than a Catch for what I was trying to do. It ties the Temple Dog to their home, without crippling them outside it. In fact, the more I think about it, the more I think it's a near perfect fit. Mouse, for example, pretty clearly takes a scene or two out to recover behind his threshold after using his Bark in White Night, in a way that's consistent with recovering from Feeding failure.
The only downside is that this is already represented a bit in the Temple Dog kit by the options for them to take Mental Stress to power their abilities. I suppose one way around this would be to switch The Bark and Sacred Guardian to Hunger stress, rather than Mental Stress, to represent them powering it from the energy they're channelling from their threshold.
I also don't know the system well enough to say what that does to balance - If you give them a feeding dependency without changing their abilities, they're going to have three stress tracks on the go in a physical encounter. It seems like it might be quite vulnerable to gaming, if done well (And YMMV as to whether that's a good thing or not).
Also, I wanted to add that I genuinely love the way that discussions about mechanics on this board have a tendency to branch out. We've got Temple Dogs, Tortoises and anti-tank weaponry, all on one page.